A system is known in which light is applied to a recording medium in the form of a sheet or a plate to form an image on a surface of the recording medium. A computer to plate (CTP) system for producing a printing plate for newspaper printing is an example of that kind of system. In the CTP system, laser light is applied to a resin layer formed on a surface of a base film or a base plate to form an image on the surface of the resin layer (expose the surface to the light).
The CTP system includes a rotating drum around which a recording medium is wrapped and an image forming apparatus that applies laser light to the recording medium wrapped around the rotating drum. The image forming apparatus includes an optical recording head capable of reciprocating along the axis of rotation of the rotating drum. When an image is formed on a recording medium by using the thus-constructed CTP system, laser light is emitted from the optical recording head of the image forming apparatus toward the recording medium surface (resin layer surface) while the rotating drum, around which the recording medium is wrapped, is being rotated. A portion of the resin layer surface is thereby exposed to the light. Subsequently, the optical recording head is moved through a predetermined distance along the axis of rotation of the drum. Thereafter, laser light is again emitted from the optical recording head toward the resin layer surface to expose a region adjacent to the region already exposed, to the light. The moving and stopping of the optical recording head is thereafter repeated to expose the entire area or a predetermined area of the resin layer surface to the light. In the following description, the direction of rotation of the rotating drum will sometimes be referred to as “main scanning direction” and the direction of movement of the optical recording head will sometimes be referred to as “sub-scanning direction”. The sub-scanning direction and the direction along the axis of rotation of the rotating drum are parallel to each other.
The conventional image forming apparatus includes at least a plurality of semiconductor lasers as a light source and a plurality of optical fibers through which laser lights emitted from the semiconductor lasers propagate.
The plurality of semiconductor lasers and the plurality of optical fibers are connected in a one-to-one relationship with each other. Laser light emitted from one of the semiconductor lasers enters the corresponding one of the optical fibers from one end (starting end) of this optical fiber and exits from the other end (terminal end) thereof. The terminal end of each optical fiber is led into the optical recording head and held in the head. More specifically, the terminal ends of the plurality of optical fibers are held in the optical recording head so that their exit end surfaces are arranged in matrix form. Laser lights respectively emitted from the plurality of exit end surfaces which are arranged in matrix form are collected on the recording medium surface through a pair of collective lenses.
It is self-evident that an image formed on the recording medium is a set of a plurality of pixels, and that one spot of laser light emitted from one optical fiber corresponds to one pixel. That is, if ten optical fibers are provided, then light exposure for ten pixels at a maximum is performed at one time. If twenty optical fibers are provided, then light exposure for twenty pixels at a maximum is performed at one time.